Whenever light passes through a prism or lens, it is dispersed or separated into all the colors which it contains, and a band of colors produced in this way is called a spectrum. If we examine such a spectrum we find the following colors in order, each color imperceptibly fading into the next: violet, indigo, blue, green, yellow, orange, red.
128. Sunlight or White Light. White light or sunlight can be dispersed or separated into the primary colors or rainbow hues, as shown in the preceding Section. What seems even more wonderful is that these spectral colors can be recombined so as to make white light.
If a prism B (Fig. 87) exactly similar to A in every way is placed behind A in a reversed position, it will undo the dispersion of A, bending upward the seven different beams in such a way that they emerge together and produce a white spot on the screen. Thus we see, from two simple experiments, that all the colors of the rainbow may be obtained from white light, and that these colors may be in turn recombined to produce white light.
FIG. 87.—Rainbow colors recombined to form white light.
White light is not a simple light, but is composed of all the colors which appear in the rainbow.
129. Color. If a piece of red glass is held in the path of the colored beam of light formed as in Section 127, all the colors on the wall will disappear except the red, and instead of a beautiful spectrum of all colors there will be seen the red color alone. The red glass does not allow the passage through it of any light except red light; all other colors are absorbed by the red glass and do not reach the eye. Only the red ray passes through the red glass, reaches the eye, and produces a sensation of color.
If a piece of blue glass is substituted for the red glass, the blue band remains on the wall, while all the other colors disappear. If both blue and red pieces of glass are held in the path of the beam, so that the light must pass through first one and then the other, the entire spectrum disappears and no color remains. The blue glass absorbs the various rays with the exception of the blue ones, and the red glass will not allow these blue rays to pass through it; hence no light is allowed passage to the eye.
An emerald looks green because it freely transmits green, but absorbs the other colors of which ordinary daylight is composed. A diamond appears white because it allows the passage through it of all the various rays; this is likewise true of water and window panes.